1. Field of the Invention
This invention relates to a control loop and method for enabling variable speed drives (VSDs) with an intermediate DC stage to ride through supply voltage sags.
2. Background of the Invention
A typical variable speed drive (VSD) includes three stages, i.e., a rectifier, an intermediate DC link, and an inverter. The rectifying stage converts the fixed-frequency, fixed-voltage input AC power from a power source such as a municipal power company into DC power. The intermediate DC link filters the DC power and has energy storage components, such as capacitors and/or inductors. Finally, the inverting stage converts the DC power into variable-frequency, variable-voltage AC power, supplied to an AC load (typically an induction motor).
The load can operate properly under given conditions only if appropriate voltage is supplied to it. The ability of a VSD to supply voltage to a load depends on the DC voltage at the DC link of the VSD. If this DC voltage decreases below a certain level, the inverter will not be able to supply the required voltage to the load. This could cause unacceptable operating conditions for the load. Unacceptable operation conditions are usually prevented by shutting the VSD down whenever an under-voltage condition is detected at the DC link. In a VSD-controlled system, such a VSD shutdown causes system downtime.
The voltage at the DC link depends on the input AC line voltage of the VSD. This input line voltage has a rated value (e.g., 460 V in the U.S.) around which it varies by a small amount (usually 10%). However, it is not uncommon that much larger temporary variations of the input line voltage occur. When these variations reduce the line voltage and have a duration ranging from half a cycle to one minute, they are called voltage sags. Voltage sags are a common cause of VSD shutdowns, since they cause the DC link voltage to drop below the minimum allowed value. The ability of a VSD to xe2x80x9csurvivexe2x80x9d (i.e., not to shut down) a voltage sag, and to resume regular operation after the voltage sag has been cleared, is called ride-through capability. Good ride-through capability is a desirable feature of any VSD.
One possible way of increasing ride-through capability of a VSD is to use an active rectifier. Such a rectifier is able, through the use of special control methods, to compensate for the variations in the input line voltage. The DC link voltage can therefore be kept at a value large enough to prevent VSD shutdowns. This is described in Annabelle van Zyl et al., Voltage Sag Ride-Through for Adjustable-Speed Drives with Active Rectifiers, 34 IEEE Transactions on Industry Applications 1270 (1998), which is incorporated herein by reference.
The most common type of rectifier used in VSDs is passive rectifier. A passive rectifier typically includes a three-phase diode bridge. With a passive rectifier, the DC link voltage is directly proportional to the input line voltage. A passive rectifier therefore does not compensate for the variations in input line voltage. Consequently, a voltage sag will cause the DC link voltage to drop, which, in turn, may cause the VSD to shutdown.
When passive rectifiers are used, one possible way of improving ride-through capability of a VSD is to provide an additional source of power connected to the DC link, as described in Annette von Jouanne et al., Assessment of Ride-Through Alternatives for Adjustable-Speed Drives, 35 IEEE Transactions on Industry Applications 908 (1999), which is incorporated herein by reference. Such an additional source of power can be provided by additional capacitors, a DC boost converter, batteries, supercapacitors, motor-generator sets, flywheels, superconductive magnetic energy storage systems, fuel cells, etc. All of these require additional hardware and therefore significantly increase the cost of a VSD.
A relatively inexpensive way of increasing the ride-through capability of a VSD with passive front end is to use the load inertia to generate power during a voltage sag (also described in Annette von Jouanne et al. cited above). In order to achieve this way of increasing the ride through capability, the inverter output frequency during a voltage sag is adjusted to a value slightly below the motor load frequency. This causes the motor to act as a generator and to maintain the DC link voltage at a desired level. This method typically requires motor speed and current sensors, which may add to the cost of a VSD.
This invention is directed to a variable speed drive comprising a rectifier configured to receive AC power from a power source and output DC power, a DC link configured to receive DC power output from the rectifier, to store some amount of DC energy, and to output DC power, an inverter configured to receive DC power output from the DC link and convert the power back to AC power and output the AC power to a motor load. The invention further includes an inverter controller operatively connected to the inverter and configured to send gating signals to the inverter, a voltage sensor configured to sense voltage applied to the rectifier and provide that sensed voltage to the inverter controller, and a sensor configured to sense at least one of power, current, and voltage of output from at least one of the DC link and the inverter and provide that sensed information to the inverter controller, wherein the inverter controller sends modified gating signals that reduce the power transferred from the inverter to the motor, when the voltage sensor senses a voltage sag.
In another aspect, the invention is also directed to a method for controlling a variable speed drive for a load, the drive including a rectifier, DC link, inverter, and an inverter control. The method comprises monitoring a voltage applied to the rectifier, monitoring at least one of power, voltage, and current output from the DC link or the inverter, controlling the inverter to provide approximately full transfer of power from the rectifier to the load as long as the voltage to the rectifier remains within a preselected range, and controlling the inverter to provide significantly lower power to the load, whenever the voltage to the rectifier falls below the preselected range.
Because the control loop and method of the present invention is based on the control of DC link power, the control loop and method do not require the sensing of motor speed and/or currents, or any additional hardware to be incorporated into a VSD. This makes it particularly suitable for low-cost variable speed drives for commercial industrial applications.
The control loop and method described herein is based on sensing the supply voltage, sensing the power flowing from the DC link to the inverter, and changing the manner in which the inverter is driven, when a sag in supply voltage is detected. The control signals to the inverter are varied, as a function of the detected voltage sag and the detected value of the power output of the DC link, which in turn is dependent upon the inherent load characteristic and operating point of the motor being driven. The generated signals are used to modify the normal control signals (such as amplitude command and frequency command) of the VSD. The modification of the control signals serves to reduce the power flowing from the DC stage to the inverter (and the inverter to the motor) to a minimum or near minimum value sufficient to keep the motor rotating and otherwise conserve the energy stored in the DC link, as long as the input voltage sag lasts. This reduced power flow is meant to prevent the DC link voltage from decreasing to a low value that would lead to a shut down. This, in turn, allows the VSD to resume regular operation after the line voltage sag is corrected.
Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.